Apparatus for molding of plastics, particularly injection molding, comprises dies mounted on die blocks, which blocks in turn are mounted on the platens of a press. Molten or plasticized material is forced by pressure out of a nozzle of a molding machine injection ram and through a mold tooling sprue bushing or the like which serves to transmit or conduct the plasticized material from the ram nozzle to the runners provided in a manifold plate. The plasticized material is then fed via runner outlet nozzles to the various individually associated mold cavities in the die blocks. These cavities are maintained at a temperature appropriate to cause solidification of the plastic formed in the die, a substantially different temperature than that of the manifold plate. In the case of thermoplastics, such mold cavities are maintained at a reduced temperature sufficient to cause solidification of the liquified thermoplastic material filling the cavity. In the case of a thermoset compound the die blocks are heated to an appropriate temperature to cause curing or “setting” of the plasticized material or compound in the die cavity after likewise being injection filled with such material in a liquid state.
“Runnerless” manifold systems are widely used in the construction of such injection molds for producing plastics and rubber parts. These manifold systems provide a method for accepting the molding material from the injection ram of the injection molding machine and distributing this material into multiple cavities or to multiple locations within the mold to produce either multiple parts simultaneously or to fill a large part mold cavity from multiple locations. In all instances, the manifold system is designed in such a way as to permit the molding material within the manifold runner passageway system to remain in its fluid plastic or uncured state such that the material remaining resident in the manifold system may be used in the next molding cycle. In this way there is no need for wastefully leaving a solid runner attached to the molded part upon demolding. For that reason, these systems are often referred to as “runnerless” molding systems. In the context of thermoplastic molding such a system also is referred to as a “Hot Runner System”. When used in the context of thermoset plastic or rubber molding, such a system is referred to as a “Cold Runner System.”
This injection molding apparatus and technology of the prior art is described hereinafter in order to highlight the difficulty associated with heating or cooling of the plastic material while it is resident in the runner channels in order to maintain the fluidity of the resident plastic material during the period of closure of the injection gate in the injection cycle thereby enabling its use in the following molding cycle.
For example, as illustrated in FIGS. 1, 2, 3, 4 and 4A, a “cold runner manifold system” is part of an injection mold assembly 20 used for the production of a rubber molded part 22. Such a cold runner system is made up of a piping or channel manifold plate 24 containing multiple channels, or “runners” 26, providing multiple flow exits through which uncured liquid rubber material flows upon being pressurefed from the outlet nozzle of an injection molding machine ram into a single manifold entrance point 28 (FIG. 1). Mold-cavity-injection nozzles 30 (FIGS. 3, 4 and 4A) are threaded into the manifold plate 24, one at each channel or runner exit.
During the injection portion of the molding cycle, these distribution channels or runners distribute the uncured rubber evenly within the mold to a number of molding cavities 32 that are configured to produce molded rubber parts 22. The manifold distribution system fills the cavities 22 of the mold 20 simultaneously under controlled pressure supplied by the injection molding machine injection ram. The temperature of this uncured rubber is held generally in the range of 50° C. while resident in the manifold distribution system. However, the cavity steel (upper and lower cavity plates 34 and 36, FIGS. 4 and 4a) of the mold is maintained at a much higher and constant cure temperature, typically within a general processing range of 160° C. As the mold cavities 32 are filled, the curing process begins. The system is thus referred to as a “cold runner system” because the system exists within a mold that is operating at a steel temperature in the realm of 160° C. while the manifold plate 24 and the rubber molding material within it is operating at a temperature in the realm of 50° C. The manifold plate runner system thus requires water cooling to maintain its lower temperature because the manifold plate runner system must operate in close proximity to, but at a significantly lower temperature than, the rest of the heated mold components (i.e., mold steel. In such a prior art system, referred to as a “cold runner system”, such cooling is provided by water channels 38 that extend roughly parallel to, or are in proximity to, the manifold rubber flow passages or runners 26.
In all instances these manifold systems require nozzles 30 at the cavity end of the runner channels 26 to facilitate, control and direct the flow of the molding material into the associated part cavities 32. These prior art nozzles 30 are conventionally formed from steel or some other high strength alloy that is highly heat conductive, and are threaded or otherwise affixed to the manifold plate 24 to bear upon mold closure on the back of the upper cavity plate 34, thereby providing a direct channel for the molding material to flow into the individual cavities 32. The nozzles are typically of either a conventional “flow through” or “valve gate” design.
Thus it will be seen that nozzles 30 used in these prior art manifold systems are located at the junction between the manifold system plate 24 and the part cavities 32 of the upper cavity plate 34 of the mold or tool 20. This location is a site where a significant temperature gradient differential occurs, i.e., as indicated above the manifold plate 24 is typically at a temperature that is 70° C. to 80° C. different from the cavity plates 34 and 36 which hold or form the molded part cavities 32.
Each nozzle 30 if not externally augmented in some way, will be influenced by temperature from the mold steel defining the part mold cavities and ultimately will achieve a temperature that will permit the fluid molding material resident in the nozzle to cure or solidify before that material is injected into the part cavities. In order to prevent this from happening, the nozzles are either heated or cooled, depending upon whether the type of injection molding application in which they are used is molding from thermoplastic or thermosetting plastic materials.
For thermoplastics molding applications, nozzles 30 are conventionally heated to roughly the same degree as the manifold system to insure that the material in the nozzle does not solidify during the cure or cooling cycle when the material in the part cavities 32 is cooled to provide the solidification necessary to produce a molded part 22. When applied to thermoset plastics or rubber molding, the nozzles 30 are cooled to roughly the same temperature as the manifold system to insure that the semi-liquid or uncured material resident in each nozzle remains in the uncured state during the mold heating or curing phase of the molding cycle when the molding material resident in the mold cavities 32 is being heat cured to provide the solidification necessary to produce a molded part 22.
Such heating or cooling of nozzles is necessary in prior art systems due to the thermal conductivity of the materials used in the construction of the prior art nozzles and the contact of each nozzle with the cavity blocks of the mold which are at significant differential temperatures with respect to the manifold. This nozzle heating takes the form of either attaching electric heaters to the O.D. of the nozzle body or installing electric heaters inside the body. As an alternative, hot oil can be circulated through the nozzle body. When nozzle cooling is required, the nozzle can be jacketed and thereby infiltrated by water channels 46 (FIG. 4A), or an array of heat pipes can be installed in the nozzle body to transfer the heat to the cold manifold plate 24 which acts as a heat sink. In both instances the complexity of the nozzle temperature augmentation system introduces unwanted equipment and maintenance costs into the system, and does not permit the use of small diameter nozzles due to space constraints.
The manifold and the nozzles described in conjunction with FIGS. 1-4A thus constitute the current design and prior art technology for a cold or hot runner manifold or system. The present method for producing these “cold runner manifold systems” is to bore the runner channels 26 into the steel manifold plate 24 to provide the rubber flow distribution passageways. The cooling water channels 38 are bored into the same plate 24 in locations roughly parallel to the rubber flow runner channels 26. All the rubber flow runner channels 26 are connected to the single sprue inlet 28 on the top face of the manifold plate 24. The multiple runner channels all exit the bottom face 40 of the manifold plate 24 in locations that correspond to the locations of the associated molding cavities 32. A nozzle 30 is threadably attached to each of the outlets of these runner channels to individually connect the same with the associated mold cavities 32. The manifold plate 24 is insulated thermally from the heated mold cavities by an insulation plate 43 that provides a thickness of insulation that has sufficient compressive strength and temperature stability to remain dimensionally stable at the elevated temperatures at which the mold operates (FIGS. 3 and 4).
The nozzles 30 (also referred to as bushings) are removably attached to the manifold plate 24, usually by threading them directly into the plate. These nozzles are installed such that they protrude through the insulation plate 43. The flat end tips 44 of the nozzles 30 bear on a mating flat portion of upper cavity plate 34 defining the margin of the associated cavity filling passageway in plate 34 such that uncured rubber exiting from each nozzle 30 is fed directly into the associated mold cavity 32 (FIG. 4A).
As noted previously, it is necessary to cool these nozzles 30 because of the metal-to-metal contact between the nozzle tip 44, the curing rubber and the upper cavity plate or steel 34. The cavity plate or steel 34 and the curing rubber are operating at temperatures in the range of 160° C., while the uncured rubber residing in nozzle 30 must be maintained at 60° C. to prevent pre-cure (“setting”) while the rubber is in the nozzle 30. The rubber residing in the manifold plate 24 is forced out of the manifold runners 26 and nozzles 30 and into the cavities 32 under the pressure generated by the injection barrel screw or piston of the molding machine. Thus, as the rubber in the cavities 32 is being cured, the rubber in residence in the nozzle 30, waiting for the next injection and cure cycle, is being adversely heated by thermal energy conducted from the upper cavity plate or steel 34 through that part of the nozzle (tip 44) that is in contact with the cavity steel 34. As illustrated by the modified nozzle 30′ of FIG. 4A, the present method for cooling these nozzles and maintaining a pre-cure temperature level for the uncured rubber resident in these nozzles is to have cooling water flow through a series of water cooling jacket channels 46 machined or cast into the nozzle body.